Comparison of the bottom nepheloid layer and late Holocene deposition on Nitinat Fan: Implications for lutite dispersal and deposition
PER R. STOKKE* I Department of Geological Sciences, Center for Marine and Environmental Studies, Lehigh University, BOBB CARSON J Bethlehem, Pennsylvania 18015 EDWARD T. BAKER* Department of Oceanography, University of Washington, Seattle, Washington 98195
ABSTRACT Suggested emplacement mechanisms for these lutites include ac- cumulation from low-density turbidity flows (Moore, 1967; A study of 56 sediment cores and 121 nephelometer profiles from Shepard and others, 1969; Piper, 1970; Nelson and Kulm, 1973), Nitinat deep-sea fan shows variations in late Holocene accumula- particle-by-particle (hemipelagic) deposition from the overlying tion rates and sediment texture which parallel variations in thick- water column (Huang and Goodell, 1970; Piper, 1970; Lisitzin, ness and suspended sediment load of the bottom nepheloid layer. 1972; Nelson and Kulm, 1973; Bouma and Hollister, 1973), or re- Furthermore, accumulation rates, sediment texture, and nepheloid deposition of winnowed sediments (Piper, 1970; Huang and layer variables all show a substantial degree of correlation with fan Goodell, 1970; Lisitzin, 1972; Bouma and Hollister, 1973; Biscaye topography. In general, the nepheloid layer thickens (>100 m) and and Eittreim, 1974). To date, no preference can be give:n to any one suspended sediment loads increase (>100 /ug/cm2) above Cascadia of these processes on modern deep-sea fans, nor is it clear what role Channel (the major channel crossing the fan) as well as above the the bottom nepheloid layer plays in the depositional process. northern flank of the fan. Over levees and the western portion of In part, the problem stems from our ignorance of the relationship the fan, the nepheloid layer thins to <50 m, and suspended sedi- between suspended matter and lutite accumulation. To assess the 2 ment loads fall below 100 /xg/cm . Cascadia Channel and the contribution of modern suspended sediments to a deep-sea fan, this northern flank of the fan have been loci of rapid sedimentation, study relates late Holocene accumulation rates, sediment texture, with accumulation rates ranging from approximately 5 to greater topography, and thickness and intensity of the nepheloid layer on 2 than 12 mg/cm /yr. In contrast, the apex and western reaches of the Nitinat Fan. While the results do not uniquely define a lutite 2 fan have significantly lower accumulation rates: 1 to 5 mg/cm /yr. emplacement mechanism, they clearly constrain the models Detailed size analysis of bottom sediments shows that areas charac- suggested. terized by rapid sedimentation and a thick, heavily loaded nepheloid layer have more medium to fine silt (5 to 7 cj>) than the TOPOGRAPHY clay-rich (>8 (f>) sediments from interchannel areas with low ac- cumulation rates and a thin, lightly loaded nepheloid layer. The Nitinat deep-sea fan is located in northern Cascadia Basin, with data suggest that turbid water moves continuously down Cascadia its apex at lat 47°55'N, long 126°30'W. It lies at the base of the Channel and the northern flank. The transport mechanism is size- continental slope off Washington and Vancouver Island, west of selective and topographically controlled, concentrating silt-sized Nitinat, Juan de Fuca, and Barkley submarine canyons (Fig. 1), detritus in topographic lows. The data also suggest a positive which apparently feed onto it. Cascadia Channel, the dominant val- downward flux of sediment particles within the nepheloid layer, at ley on the fan, issues from the joint terminus of Nitinat and Juan de least when averaged over a significant period of time. Fuca Canyons and trends southward along the margin of the slope. The channel widens just south of the apex to form a series of sub- INTRODUCTION channels, before assuming a distinct V-shape farther down channel (at lat 47°38'N). To the west and north, the channel is bordered by Deep-sea fans are major sedimentary features off some continen- a well-developed natural levee with a relief of 50 to 75 m (at depths tal margins. As such, they have been the subject of numerous less than 2,400 m; Fig. 1). studies (Gorsline and Emery', 1959; Shepard and others, 1969; In contrast, the northern flank of the fan is relatively featureless Normark, 1970; Piper, 1970; Curray and Moore, 1971; Normark and has no apparent connection to the canyons egressing at the and Piper, 1972; Hein, 1973; and others) describing bathymetric apex. Barkley Canyon appears to open onto the northern flank, but development, dispersal patterns, and rates of accumulation. These no associated channel is observed on the fan. investigations have emphasized turbidity currents as the primary The southwestern portion of the fan displays a complex topog- mechanism of transportation and deposition of sand and sandy silt. raphy with several small, discontinuous valleys. Some of these cross In contrast, relatively little attention has been paid to the the fan and feed into Cascadia Channel south of lat 46°30'N. processes controlling dispersal and accumulation of fine-grained Others can be traced only a few kilometres and are, apparently, sediments (clayey silt and clay) on deep-sea fans. Whereas some remnants of channel migration. material is undoubtedly transported by turbidity currents, a sig- nificant portion shows no evidence of turbidite deposition, particu- METHODS larly in sediments deposited during the past 9,000 to 12,000 yr (Shepard and others, 1969; Piper, 1970; Carlson and Nelson, Data and samples for this study were collected during three 1969; Huang and Goodell, 1970). cruises from 1971 to 1974. During these cruises, 121 nephelometer 5 Present addresses: (Stokke) Continental Shelf Institute, Hakon Mag- profiles were recorded in the waters over the fan. The integrating nussonsgt. IB, 7000 Trondheim, Norway. (Baker) Pacific Marine Environ- nephelometer used in the study was described by Sternberg and mental Laboratory, NOAA, 3711 15th Avenue NE, Seattle, Washington others (1974). 98105. Recognition of the precise top of the bottom nepheloid layer is
Geological Society of America Bulletin, v. 88, p. 1586-1592, 12 figs., November 1977, Doc. no. 71106.
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127° 30' 127° 00' 126° 30' 126° 00' 127° 30' 127" 00' 126° 30' 126*00'
Figure 1. Study area in the northeast Pacific Ocean, bathymetry of Nitinat Fan, and locations of the cross sections in Figure 5.
somewhat subjective. In this study, the nepheloid layer is opera- RESULTS tionally defined as the zone of increasing light-scattering extending from the deepest depth of clearest water to the sea floor. The scat- Nepheloid Layer tering intensity within this layer can be related to the mass of sedi- ment suspended in the nepheloid layer per unit cross section of the Figures 2 and 3 are composite maps of the nepheloid layer thick- water column, a measure obtained by calibration of the relation of ness and suspended sediment load from three cruises (1971 to the scattering intensity and suspensate concentration and integra- 1974). The primary characteristics of the nepheloid layer remained tion of the appropriate area under the nephelometer profile. stable over this period, and, hence, composite maps can reasonably The study of bottom sediments is based on 56 gravity cores. Ac- be drawn (Baker and others, 1974; Baker, 1976). cumulation rates (in mg/cm2/yr) were calculated according to the The thickness of the nepheloid layer (Fig. 2) is in general con- method of Koczy (1951). Results are based on a previously defined trolled by the topography of the fan, since the regional trend of the stratigraphy and an average sediment density of 2.64 g/cm3. top of the nepheloid layer is a gradual seaward deepening. The The time stratigraphy used is based on faunal analysis (plankton nepheloid layer is thickest (>100 m) over both Cascadia Channel formainifera/radiolarian ratio) and 16 radiocarbon dates (Carson, and the steeply dipping northwestern flank of the fan. Within Cas- 1971). Stratigraphic units (early Holocene, 13,000 to 9000 B.P.; cadia Channel, the nepheloid layer gradually thickens downchan- late Holocene, 9000 B.P. to present) were defined by a modified ac- nel, culminating in an abrupt increase (to >200 m) where the ceptance sampling procedure.(Carson and McManus, 1971). channel becomes sharply constricted (near lat 47°38'N). Over the Size analyses (0
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1 127° 30' 127° 00 126° 30' 126° OO" 1270 30' 127° 00' 126" 30' 126° 00' 48° 30' 48° 30'
48° 00' - _ 48° 00' •
LATE HOLOCENE 47° 30' _ 47° 30 ACCUMULATION RATES (mg/cm2- yr)
47° 00' 47° 00' Figure 3. Scattering intensity of the bottom nepheloid layer (BNL) ex- Figure 4. Late Holocene (0 to 9000 B.P.) accumulation rates (mg/cm2/ pressed as the mass of sediment suspended within a 1-cm2 column of water yr). Solid symbols indicate samples from Cascadia Channel, secondary extending from the top of the nepheloid layer to the sea floor. channels, and northern flank of Nitinat Fan; open circles are samples from interchannel regions to the west and southwest (separation defined in Fig. 9). Squares represent samples with anomalously low accumulation rates Channel and the northwestern flank. Loadings in Cascadia Chan- (see Fig. 12). nel increase downchannel, with generally higher values found along the western half, until an abrupt increase (to >200 /¿.g/cm2) at the point of constriction of the channel. As with the thickness values, cores west of the levee close to the apex, which are associated both there is a lobe of high sediment load over the northwestern flank with relict channels in this area and with the thick and heavily which curves around the corner of the levee to the south. Loadings loaded lobe of the nepheloid layer, which apparently curves around over the levee crest near the fan apex are typically <50 /u,g/cm2, the corner of the levee. whereas loadings over most of the southwestern flank are 50 to 100 In order to compare more closely the nepheloid layer and as- 2 /¿g/cm . Data control around the levee south of lat 47°43'N is poor, sociated accumulation rates, five cross sections were drawn however, and it is difficult to ascertain whether the areas of high through the upper fan (Figs. 1, 5). These sections indicate that suspended sediment loadings to the west of the levee are isolated although the regional trend of the nepheloid layer surface is a and perhaps transient features or are related to a relatively perma- gradual seaward deepening that trend is consistently modified, nent "overflow" of material from Cascadia Channel. presumably in response to the fan topography, to produce a nepheloid layer that displays a relatively uniform (when averaged over 4 y of measurement) thickness across Cascadia Channel Accumulation Rates (profiles B, C, and D, Fig. 5). Seaward of the levee, the surface of the nepheloid layer deepens again, and the layer becomes generally Late Holocene (0 to 9000 B.P.) accumulation rates (Fig. 4) show thinner, except over the northwest flank. Variations in nepheloid a clear relationship with the fan topography and follow closely the layer thickness and suspended sediment load are paralleled re- trends established by the variables of the nepheloid layer. Cascadia markably well by late Holocene and even early Holocene accumu- Channel and the northern flank of the fan have been loci of rapid lation rates. Indeed, a significant positive correlation exists between sedimentation, with accumulation rates ranging from approxi- late Holocene accumulation rates and nepheloid layer thickness (r 2 mately 5 to greater than 12 mg/cm /yr. In contrast, the levee crest = 0.71; Fig. 6) and, even more pertinently, between late Holocene and western interchannel reaches of the fan have significantly lower accumulation rates and the suspended sediment load in the 2 accumulation rates — 1 to 5 mg/cm /yr. nepheloid layer (r = 0.65; Fig. 7). Within Cascadia Channel, the highest accumulation rates are found along the western margin and at the point where the channel Textural Variations becomes sharply constricted (near lat 47°40'N), the same pattern observed in the nepheloid layer suspended sediment loads. Moder- To ascertain whether textural variations in bottom sediments are 2 ate accumulation rates (2.5 to 7.5 mg/cm /yr) are found in a few associated with the distributions of accumulation rates and charac-
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-19 19.
.20 20.
-21 21.
-22 22.
.23 23.
BNL SUSPENDED SEDIMENT LOAD (jjg/cm2) OTOP OF BNL. [lOmg/crr? yr <50 H 150-200 • L.H. ACC. RATES 50-100 ^ >200 Hi O E.H. ACC. RATES 100-150
Figure 5. Cross sections through the bottom nepheloid layer (BNL) and Nitinat Fan, showing the relationship between the top of the nepheloid layer, the underlying bathymetry, and late Holocene and early Holocene accumulation rates. Suspended sediment load is integrated from the top of the nepheloid layer down, so that the total load at a given station (as shown in Fig. 3) is indicated by the load immediately above the sea floor. Location of cross sections is given in Figure 1.
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the middle to lower fan farther to the west, lower measures (<0.25, finer sediments) are observed. Figure 6. Bottom A more detailed study of the data reveals considerable variation nepheloid layer (BNL) within the broad, upper portion of Cascadia Channel, indicating a thickness versus late variety of textures. Coarser material is preferentially located in the Holocene accumulation western portion of the channel, just as are the heaviest suspended rates. Regression line: sediment loads in the nepheloid layer. Furthermore, coarse sedi- nepheloid layer thickness = 10.4 late Holocene accumu- ments (factor one measures = 0.25 to 0.75) are found in the relict lation rates + 31.8, r = channels west of the levee at the apex. Internal contouring of the 0.71, F(1>35) = 36.09. Sym- area of the fan with measures <0.25 (not shown here) indicates bols same as those used in that the sediments decrease in coarseness away from (west of) the Figure 4. 6 8 apex of the fan and from Cascadia Channel. mg/cm2xyr While factor measures are useful in defining and describing sev- LATE HOLOCENE ACCUMULATION RATES eral (key) size classes as single parameters, it is instructive (and perhaps reassuring) to examine the original size distributions (Fig. teristics of the nepheloid layer, size distributions were examined 11). There is, indeed, a pronounced difference between sample statistically. An R-mode factor analysis defined four factors (linear groups (as defined by the cluster analysis) from interchannel and combinations of the percentage of total weight in each 250_ ÜJ DISCUSSION O 150 UJ Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/11/1586/3429189/i0016-7606-88-11-1586.pdf by guest on 27 September 2021 BOTTOM NEPHELOID LAYER AND LATE HOLOCENE DEPOSITION ON NITINAT FAN 1591 «•V /ÄEV Figure 9. Factor mea- , ,6502.: Vf¡30>i-. sures for factor one plotted \ / /f^f • 630VVV r^-pS versus measures for factor two. Symbols same as those used in Figure 4. 9017 .4 • CHANNEL OR NORTH SLOPE fine-grained detritus on Nitinat Fan. The characteristics are spec- ifically defined as follows. :0 1. The distribution of light-scattering results suggests that tur- 6J 6302 A 3 6303 J 6304 A J 6S06 bid, albeit low-density, water emanates from Nitinat, Juan de Fuca, and Barkley Canyons and, influenced by the topography of the upper fan, moves primarily down Cascadia Channel and the north- ern flank, respectively. Such turbid water dispersal may be analo- gous to the low-density turbidity flows postulated by Moore (1967) for the San Diego Trough. These "flows," however, are not ULiLlLi episodic (as one might expect by analogy with late Pleistocene tur- bidity currents on Nitinat Fan), but reasonably continuous, as evi- denced by the "permancency" of the configuration of the nepheloid layer from 1971 to 1974. Continuous deposition from such 127° 30' 127° 00' 126° 30' 126* 00' 48° 30' r^ Figure 11. Histograms of size distributions from interchannel areas, from Cascadia Channel, and from northern flank of Nitinat Fan. CO LU < 5 o (FACTOR I MEASURES) : < 0.25 .2 .4 TEXTURE, FACTOR I —»• COARSER 1 0.25-0.50 Figure 12. Late Holocene (0 to 9000 B.P.) accumulation rates plotted versus texture (factor one) of the bottom sediments. Solid line is regression 0.50 - 0.75 1 line for open and closed circles: late Holocene accumulation rates = 13.3 1 0.75 -1.00 texture + 1.2, r = 0.675, F(lj 34) = 26.8. Dotted lines are computed from discriminant functions and represent the best separations between respec- 47° 00 tive groups. Line separating solid squares: late Holocene accumulation rates Figure 10. Textural variations illustrated as a contour plot of factor = 15.8 texture - 2.9, d = 7.4, F<2, 431 = 32.2. Line separating open circles: measures for factor one. Symbols used for core locations are the same as late Holocene accumulation rates = 63.6 texture + 19.0, d = 7.6, F(2,43l = those used in Figure 4. 39.0. Symbols same as those used in Figure 4. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/11/1586/3429189/i0016-7606-88-11-1586.pdf by guest on 27 September 2021 1588 STOKKE AND OTHERS "flows" would account for the total lack of layering or characteris- Biscaye, P. E., and Eittreim, S. L., 1974, Variations in benthic boundary tic "turbidite structures" in the late Holocene section. layer phenomena: Nepheloid layer in the North American Basin, in Near-bottom increases of suspended sediment concentration are Gibbs, R. J., ed., Suspended solids in water: New York, Plenum Press, obviously not restricted to Cascadia Channel and the northwestern p. 227-260. flank. Maps of nepheloid layer thickness and suspended sediment Bouma, A. H., and Hollister, C. D., 1973, Deep ocean basin sedimentation: Soc. Ecom. Paleontogists and Mineralogists Pacific Sec., Short course load (Figs. 2, 3) suggest transport routes to the "open" portions of lecture notes, Anaheim 1973, Turbidites and deep water sedimenta- the fan. For instance, the lobe of thick, turbid water that curves tion, p. 79-118. around the corner of the levee (Figs. 2, 3) suggests dispersal by a Carlson, P. R., and Nelson, C. H., 1969, Sediments and sedimentary struc- regional bottom current flowing south along the base of the slope tures of the Astoria submarine canyon-fan system, northeast Pacific: and being deflected by the apex of the fan. Similarly, the high sus- Jour. Sed. Petrology, v. 39, p. 1269-1282. pended sediment loading found to the west of the levee south of lat Carson, B., 1971, Stratigraphy and depositional history of Quaternary sed- 47°45'N may represent "overflow" from the western half of Cas- iments in northern Cascadia Basin and Juan de Fuca Abyssal Plain, cadia Channel. northeast Pacific Ocean [Ph.D. dissert.]: Seattle, Univ. Washington, 248 p. 2. The transport mechanism is size-selective, concentrating Carson, B., and McManus, D. A., 1971, Analysis of turbidite correlation in silt-sized detritus in topographic lows. The enrichment of silt, rela- Cascadia Basin, northeast Pacific Ocean: Deep-Sea Research, v. 18, tive to clay sizes, in Cascadia Channel and on the northern flank p. 593-604. may result either from winnowing of the finer (>8 Baker, E. T., 1976, Temporal and spatial variability of the bottom nepheloid layer over a deep-sea fan: Marine Geology, v. 21, p. 67-79. Baker, E. T., Sternberg, R. W., and McManus, D. A., 1974, Continuous light-scattering profiles and suspended matter over Nitiant deep-sea MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 3, 1976 fan, in Gibbs, R. J., ed., Suspended solids in water: New York, Plenum REVISED MANUSCRIPT RECEIVED SEPTEMBER 16, 1976 Press, p. 155-172. MANUSCRIPT ACCEPTED NOVEMBER 30, 1976 Printed in U.S.A. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/11/1586/3429189/i0016-7606-88-11-1586.pdf by guest on 27 September 2021